I/O path optimization based on cache slot location

ABSTRACT

Processing an I/O operation may include the host selecting one of the available paths over which to send each I/O operation to the data storage system. The selected path may be to a particular director that has responsibility for cache slot allocation and locally accessing the cache slot predicted to include the data of the I/O operation. The host may understand the cache slot allocation algorithm used on the data storage system and how cache slots are allocated for particular logical devices and tracks or locations on the logical devices. The host may direct I/Os down a path to a particular director that has, or will allocate, the cache slot used for the I/Os. There may be multiple directors in a data storage system including a distributed global memory. Each director may locally access a group of cache slots and communicate over a fabric to access the distributed global memory.

BACKGROUND Technical Field

This application generally relates to data storage and more particularly to techniques used in connection with I/O (input/output) operations.

Description of Related Art

Computer systems may include different resources used by one or more host processors. Resources and host processors in a computer system may be interconnected by one or more communication connections. These resources may include, for example, data storage devices such as those included in the data storage systems manufactured by EMC Corporation. These data storage systems may be coupled to one or more servers or host processors and provide storage services to each host processor. Multiple data storage systems from one or more different vendors may be connected and may provide common data storage for one or more host processors in a computer system.

A host processor may perform a variety of data processing tasks and operations using the data storage system. For example, a host processor may perform I/O (input/output) operations in connection with data requests, such as data read and write operations.

Host processor systems may store and retrieve data using a storage device containing a plurality of host interface units, disk drives, and disk interface units. The host systems access the storage device through a plurality of channels provided therewith. Host systems provide data and access control information through the channels to the storage device and the storage device provides data to the host systems also through the channels. The host systems do not address the disk drives of the storage device directly, but rather, access what appears to the host systems as a plurality of logical disk units. The logical disk units may or may not correspond to the actual disk drives. Allowing multiple host systems to access the single storage device unit allows the host systems to share data in the device. In order to facilitate sharing of the data on the device, additional software on the data storage systems may also be used.

Data storage systems, hosts and other components may be interconnected by one or more communication connections such as in a network configuration. The network may support transmissions in accordance with well-known protocols such as TCP/IP (Transmission Control Protocol/Internet Protocol), UDP (User Datagram Protocol), and the like. Networked storage systems, such as data storage arrays, may be used to maintain data on different systems in different locations.

SUMMARY OF THE INVENTION

In accordance with one aspect of techniques herein is a method of processing an I/O operation comprising: receiving, on a host, the I/O operation directed to a target location of a logical device having storage provisioned on a data storage system; predicting, in accordance with a cache slot allocation algorithm used by the data storage system, a first cache slot of a cache of the data storage system, wherein the first cache slot is expected to be used to store cached data for the target location of the logical device; determining, by the host, a director of the data storage system, wherein the director locally accesses the first cache slot of the cache of the data storage system; and sending the I/O operation from the host to the data storage system over a path, said path being a physical connection from the host to the director of the data storage system. The predicting and the determining may be performed by an I/O driver of the host. The target address may denote a logical address or location on the logical device. The cache slot allocation algorithm may map the logical device and logical address (or location on the logical device to which the I/O operation is directed) to the first cache slot, and the first cache slot may be included in a first global memory portion that is local with respect to the director. The data storage system may include a plurality of directors comprising the director. The data storage system may further include a distributed global memory comprising a plurality of global memory portions including the first global memory portion. Each of the plurality of directors may locally access a different one of the plurality of global memory portions, and the cache of the data storage system may comprise groups of multiple cache slots, where each of the groups of multiple cache slots may be included in a different one of the global memory portions. Each of the plurality of directors may be configured to communicate over a fabric to access the distributed global memory. Each of the plurality of directors may be configured to send all requests to access the distributed global memory if processor utilization of said each director exceeds a maximum threshold utilization level. The method may include determining, by a first of the plurality of directors, that processor utilization of said first director exceeds the maximum threshold utilization level; and responsive to determining that processor utilization of the first director exceeds the maximum threshold utilization level, issuing a request by said first director over the fabric to access first data stored in a first location of one of the plurality of global memory portions that is locally accessible to the first director. The method may include returning, over the fabric to said first director, first data stored in the first location of the one global memory portion that is locally accessible to the first director. Each of the plurality of directors may be configured to send, over the fabric, requests that transfer an amount of data exceeding a defined maximum amount, wherein the requests may access locations in the distributed global memory. The method may include determining, by a first of the plurality of directors, that a first request to access a first location in the distributed global memory transfers a first amount of data exceeding the defined maximum amount, wherein the first location denotes a global memory location of the distributed global memory and the global memory location is locally accessible to the first director; and responsive to determining that the first amount exceeds the defined maximum amount, issuing the first request by the first director over the fabric. The method may include returning, over the fabric to the first director, first data stored in the first location denoting the global memory location. The steps of predicting, determining and sending may be performed responsive to the I/O operation being a request that reads or writes a first amount of data that does not exceeds a maximum size. Responsive to determining the I/O operation exceeds a maximum size, the I/O operation may be sent to any one of a plurality of directors of the data storage system selected independent of which particular director of the plurality of directors is predicted by the cache slot allocation algorithm to locally access the first cache slot. The method may include determining whether the I/O operation accesses data spanning multiple cache slots of the cache of the data storage system, wherein each of the multiple cache slots is locally accessible to a different one of the plurality of directors; responsive to determining the I/O operation accesses data spanning multiple cache slots each locally accessible to a different one of the plurality of directors, partitioning the I/O operation into multiple I/O operations wherein each of the multiple I/O operations accesses data of a different one of the multiple cache slots; and issuing each of the multiple I/O operations accessing said different one of the multiple cache slots over a particular path to one of the plurality of directors that locally accesses said different one of the multiple cache slots.

In accordance with another aspect of the techniques herein is a computer readable medium comprising code stored thereon that, when executed, performs method of processing an I/O operation comprising: receiving, on a host, the I/O operation that reads or writes first metadata of a logical device having storage provisioned on a data storage system; predicting, in accordance with an algorithm used by the data storage system, a first cache slot of a cache of the data storage system, wherein the first cache slot is expected to be used to store the first metadata for the logical device; determining, by the host, a director of the data storage system, wherein the director locally accesses the first cache slot of the cache of the data storage system; and sending the I/O operation from the host to the data storage system over a path, said path being a physical connection from the host to the director of the data storage system. The data storage system may include a plurality of directors comprising the director, and the data storage system may further include a distributed global memory comprising a plurality of global memory portions including a first global memory portion that is locally accessible to the director. The first global memory portion may include the first cache slot, and each of the plurality of directors may locally access a different one of the plurality of global memory portions. The cache of the data storage system may comprise groups of multiple cache slots, and each of the groups of multiple cache slots may be included in a different one of the global memory portions.

In accordance with another aspect of techniques herein is a system comprising: a processor; and a memory comprising code stored thereon that, when executed, performs a method of processing an I/O operation comprising: receiving, on a host, the I/O operation that reads or writes first data stored on a data storage system; predicting, in accordance with an algorithm used by the data storage system, a first cache slot of a cache of the data storage system, wherein the first cache slot is expected to be used to store the first data; determining, by the host, a director of the data storage system, wherein the director locally accesses the first cache slot of the cache of the data storage system; and sending the I/O operation from the host to the data storage system over a path, said path being a physical connection from the host to the director of the data storage system. The data storage system may include a plurality of directors comprising the director, and the data storage system may include a distributed global memory comprising a plurality of global memory portions including a first global memory portion that is locally accessible to the director. The first global memory portion may include the first cache slot, and each of the plurality of directors may locally access a different one of the plurality of global memory portions. The cache of the data storage system may comprise groups of multiple cache slots, and each of the groups of multiple cache slots may be included in a different one of the global memory portions.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the present invention will become more apparent from the following detailed description of exemplary embodiments thereof taken in conjunction with the accompanying drawings in which:

FIGS. 1, 3 and 4 are examples of embodiments of systems and components that may utilize the techniques described herein;

FIG. 2A is an example of an embodiment of a data storage system;

FIG. 2B is a representation of the logical internal communications between the directors and memory included in one embodiment of the data storage system of FIG. 2A;

FIG. 5 is an example of a command request that may be used in an embodiment in accordance with techniques herein;

FIG. 6 is an example of directors that may be included in a data storage system in an embodiment in accordance with techniques herein;

FIG. 7 is an example of a memory map of a director in an embodiment in accordance with techniques herein;

FIG. 8 is an example illustrating a logical representation of cached data portions collectively stored in the global memory portions of the directors of a data storage system in an embodiment in accordance with techniques herein;

FIG. 9 is an example of information that may be used by an I/O driver of the host in an embodiment in accordance with techniques herein; and

FIG. 10 is a flowchart of processing steps that may be performed in an embodiment in accordance with techniques herein.

DETAILED DESCRIPTION OF EMBODIMENT(S)

Referring to FIG. 1, shown is an example of an embodiment of a system and components that may be used in performing the techniques described herein. The system 10 includes a data storage system 12 connected to host systems 14 a-14 n through communication medium 18. In this embodiment of the system 10, the N hosts 14 a-14 n may access the data storage system 12, for example, in performing input/output (I/O) operations or data requests. The communication medium 18 may be any one or more of a variety of networks or other type of communication connections as known to those skilled in the art. The communication medium 18 may be a network connection, bus, and/or other type of data link, such as a hardwire or other connections known in the art. For example, the communication medium 18 may be the Internet, an intranet, network or other wireless or other hardwired connection(s) by which the host systems 14 a-14 n may access and communicate with the data storage system 12, and may also communicate with others included in the system 10.

Each of the host systems 14 a-14 n and the data storage system 12 included in the system 10 may be connected to the communication medium 18 by any one of a variety of connections as may be provided and supported in accordance with the type of communication medium 18. The processors included in the host computer systems 14 a-14 n may be any one of a variety of proprietary or commercially available single or multi-processor system, such as an Intel-based processor, an IBM mainframe computer system (e.g., such as the z/Architecture as described in more detail elsewhere herein), or other type of commercially available processor able to support traffic in accordance with each particular embodiment and application.

It should be noted that the particulars of the hardware and software included in each of the components that may be included in the data storage system 12 are described herein in more detail, and may vary with each particular embodiment. Each of the host computers 14 a-14 n and data storage system may all be located at the same physical site, or, alternatively, may also be located in different physical locations. Examples of the communication medium that may be used to provide the different types of connections between the host computer systems and the data storage system of the system 10 may use a variety of different communication protocols such as SCSI, ESCON, Fibre Channel, FICON, iSCSI, or GIGE (Gigabit Ethernet), and the like. Some or all of the connections by which the hosts and data storage system 12 may be connected to the communication medium 18 may pass through other communication devices, such as switching equipment, a phone line, a repeater, a multiplexer or even a satellite.

Each of the host computer systems may perform different types of data operations in accordance with different tasks and applications executing on the hosts. In the embodiment of FIG. 1, any one of the host computers 14 a-14 n may issue a data request to the data storage system 12 to perform a data operation. For example, an application executing on one of the host computers 14 a-14 n may perform a read or write operation resulting in one or more data requests to the data storage system 12.

Referring now to FIG. 2A, shown is an example of an embodiment of the data storage system 12 that may be included in the system 10 of FIG. 1. Included in the data storage system 12 of FIG. 2A are one or more data storage systems 20 a-20 n as may be manufactured by one or more different vendors. Each of the data storage systems 20 a-20 n may be inter-connected (not shown). Additionally, the data storage systems may also be connected to the host systems through any one or more communication connections 31 that may vary with each particular embodiment and device in accordance with the different protocols used in a particular embodiment. The type of communication connection used may vary with certain system parameters and requirements, such as those related to bandwidth and throughput required in accordance with a rate of I/O requests as may be issued by the host computer systems, for example, to the data storage system 12. In this example as described in more detail in following paragraphs, reference is made to the more detailed view of element 20 a. It should be noted that a similar more detailed description may also apply to any one or more of the other elements, such as 20 n, but have been omitted for simplicity of explanation. It should also be noted that an embodiment may include data storage systems from one or more vendors. Each of 20 a-20 n may be resources included in an embodiment of the system 10 of FIG. 1 to provide storage services to, for example, host computer systems.

Each of the data storage systems, such as 20 a, may include a plurality of data storage devices, such as disk devices or volumes (more generally physical storage devices), in an exemplary arrangement 24 consisting of n rows of disks or volumes 24 a-24 n. In this arrangement, each row of disks or volumes may be connected to a disk adapter (“DA”) or director responsible for the backend management of operations to and from a portion of the disks or volumes 24. In the system 20 a, a single DA, such as 23 a, may be responsible for the management of a row of disks or volumes, such as row 24 a. The system 20 a may also include one or more host adapters (“HAs”) or directors 21 a-21 n. Each of these HAs may be used to manage communications and data operations between one or more host systems and the global memory. In an embodiment, the HA may be a Fibre Channel Adapter (FA) or other, more generally, other front end adapter which facilitates host communication.

Also shown in the storage system 20 a is an RA or remote adapter 40. The RA may be hardware including a processor used to facilitate communication between data storage systems, such as between two of the same or different types of data storage systems. In one embodiment described in more detail in following paragraphs and figures, the RAs of the different data storage systems may communicate over Fibre Channel transmission channel supporting messaging traffic between data storage systems. The RA may be hardware including a processor used to facilitate communication between data storage systems, such as between two Symmetrix® data storage systems. The RA may be used with the Symmetrix Remote Data Facility (SRDF®) products provided by EMC Corporation of Hopkinton, Mass. SRDF® is a family of products that facilitates the data replication from one Symmetrix® storage array to another through a Storage Area Network (SAN) or and IP network. SRDF® logically pairs a device or a group of devices from each array and replicates data from one to the other synchronously or asynchronously. Generally, the SRDF® products are one example of commercially available products that may be used to provide functionality of a remote data facility (RDF) for use in an embodiment in connection with techniques herein.

One or more internal logical communication paths may exist between the DA's, the RA's, the HA's, and the memory 26. An embodiment, for example, may use one or more internal busses and/or communication modules. For example, the global memory portion 25 b may be used to facilitate data transfers and other communications between the DA's, HA's and RA's in a data storage system. In one embodiment, the DAs 23 a-23 n may perform data operations (e.g., read and write I/O operations) using a cache that may be included in the global memory 25 b, for example, in communications with other disk adapters or directors, and other components of the system 20 a. The other portion 25 a is that portion of memory that may be used in connection with other designations that may vary in accordance with each embodiment.

It should be generally noted that the elements 24 a-24 n denoting data storage devices may be any suitable physical storage device such as a rotating disk drive, flash-based storage, and the like. The particular data storage system as described in this embodiment, or a particular device thereof, such as a rotating disk or solid state storage device (e.g., a flash-based storage device), should not be construed as a limitation. Other types of commercially available data storage systems, as well as processors and hardware controlling access to these particular devices, may also be included in an embodiment.

Host systems provide data and access control information through channels to the storage systems, and the storage systems may also provide data to the host systems also through the channels. The host systems do not address the disk drives of the storage systems directly, but rather access to data may be provided to one or more host systems from what the host systems view as a plurality of logical devices or logical volumes. For example, one or more logical devices or volumes may reside on a single physical disk drive. Data in a single storage system may be accessed by multiple hosts allowing the hosts to share the data residing therein. The HAs may be used in connection with communications between a data storage system and a host system. The RAs may be used in facilitating communications between two data storage systems. The DAs may be used in connection with facilitating communications to the associated physical storage devices or drive(s) and logical devices residing thereon.

I/O requests, such as read and write requests sent from a host to the data storage system, may be directed to a target address, such as a logical device and location on the logical device. The target address may be mapped or translated into a corresponding physical storage location on physical storage storing the data for the corresponding target address. Such I/O requests from the host may be received by a front end director or adapter, such as the HA or FA described herein. The receiving FA may perform processing to service the I/O operation. Global memory, or a subset thereof, on the data storage system, may be used as the data cache (also referred to as simply as a cache herein). In at least one embodiment, write data received at the data storage system from a host or other client may be initially written to the cache (e.g., cache memory such as may be included in the component designated as 25 b) and marked as write pending (WP). Once written to cache, the host may be notified that the write operation has completed. At a later point time, the write data marked as WP may be destaged from cache to the physical storage device, such as by a DA. If the I/O operation is a read request to read data from a logical device location, processing is performed to determine whether the requested read data is already stored in cache thereby resulting in a read hit. If there is a read hit, the FA may retrieve the requested read data from cache and return the requested read data to the host or other client that issued the read. If the read data is not stored in cache resulting in a read miss, a request is issued to retrieve the requested read data from physical storage. Data that is read from physical non-volatile storage devices, such as back-end physical storage devices accessed by the DAs, is then stored in the cache. The FA may retrieve the requested read data now stored in cache and return the requested read data to the host or other client that issued the read.

It should be noted that data storage system global memory, such as denoted by 25 b in FIG. 2A, may denote a logical representation of global memory. As described in more detail elsewhere herein, the global memory of the data storage system, such as used in connection with data caching for I/O operations, may be implemented as a distributed global memory with different portions of the global memory local to different corresponding directors or adapters. In such an embodiment, all portions of the global memory may be generally accessible to all directors. Particular one or more portions of the global memory may be local with respect to a particular director with remaining non-local portions of the global memory accessible to the particular director using a communications fabric, such as an Infiniband (IB) fabric. The foregoing as may be included in at least one embodiment of techniques herein is described in more detail below.

Referring to FIG. 2B, shown is a representation of the logical internal communications between the directors and memory included in a data storage system. Included in FIG. 2B is a plurality of directors 37 a-37 n coupled to the memory 26. Each of the directors 37 a-37 n represents one of the HA's, RA's, or DA's that may be included in a data storage system. The representation of FIG. 2B also includes an optional communication module (CM) 38 that provides an alternative communication path between the directors 37 a-37 n. Each of the directors 37 a-37 n may be coupled to the CM 38 so that any one of the directors 37 a-37 n may send or receive a message and/or data to any other one of the directors 37 a-37 n without needing to go through the memory 26. The CM 38 may be implemented using conventional MUX/router technology where a sending one of the directors 37 a-37 n provides an appropriate address to cause a message and/or data to be received by an intended receiving one of the directors 37 a-37 n. In addition, a sending director 37 a-37 n may be able to broadcast a message to all of the other directors 37 a-37 n at the same time.

In an embodiment in accordance with techniques herein, the data storage system as described may be characterized as having one or more logical mapping layers in which a logical device of the data storage system is exposed to the host whereby the logical device is mapped by such mapping layers of the data storage system to one or more physical devices. Additionally, the host may also have one or more additional mapping layers so that, for example, a host side logical device or volume is mapped to one or more data storage system logical devices as presented to the host.

Referring to FIG. 3, shown is an example of an embodiment of a system 100 that may be used in connection with the techniques described herein. It should be noted that the embodiment illustrated in FIG. 3 presents a simplified view of some of the components illustrated in FIGS. 1 and 2A, for example, including only some detail of the data storage systems 20 a through 20 n for the sake of illustration.

Included in the system 100 are data storage systems 102 and 104 and hosts 110 a, 110 b and 110 c. Hosts 110 a, 110 b and 110 c may perform operations to data storage system 102 over connection 108 a. Also hosts 110 a, 110 b and 110 c may perform operations, such as I/O operations, to data storage system 104 over connection 108 b. The hosts 110 a, 110 b and 110 c may be connected to the data storage systems 102, 104 through connections 108 a, 108 b which may be, for example, network or other type of communication connection. Although not illustrated, the hosts 110 a-110 c may also be directly connected to a network such as the Internet.

The data storage systems 102 and 104 may include one or more logical devices. In this example, data storage system 102 includes logical device L1 124 and data storage system 104 includes logical device L2 126. Both of the data storage systems may include one or more other logical and/or physical storage devices.

In at least one embodiment, one or more of the hosts 110 a-110 c may access logical device L1 124 over connection 108 a, and one or more of the hosts 110 a-110 c may access logical device L2 126 over connection 108 b. Both the logical devices L1 124 and L2 126 may have a data layout based on the CKD (count key data) architecture and format, and one or more of the hosts 110 a-c may access data of devices L1 124 and L2 126 using a FICON connection. IBM's FICON (Fiber Connection) is a Fibre Channel protocol that provides high-speed connectivity between a channel and a control device and allows multiple data exchanges in full duplex mode. FICON may be used with Fibre Channel communication. FICON is compatible with z/Architecture computing systems in connection with I/O devices performing I/O processing therewith.

Elements 102 and 104 may each be a Symmetrix® data storage system, provided by EMC Corporation of Hopkinton, Mass., which is a data storage system compatible with FICON. For further discussion of FICON in connection with IBM System/390, a precursor to IBM System z, see DeCusatis, et al., “Fiber optic interconnects for the IBM S/390 Parallel Enterprise Server G5,” IBM J. Res. Develop., Vol. 43, No. 5/6, September/November 1999, pp. 807-828, which is incorporated herein by reference.

CKD may be characterized as a data storage device architecture where each storage device record includes of a count field, an optional key field, and a (“user”) data field with error correction/detection information appended to each field. The sizes of the records within a single device may vary with the CKD architecture. Since data record lengths can vary, they all have an associated count field which indicates the size of the key if used and the size of the data. The count field has the identification of the physical location in cylinder-head-record format, the length of the key, and the length of the data. The key may be omitted or consist of a string of characters. Most often the key is omitted, the record located sequentially or by direct cylinder-head-record addressing. If it is present, the key is typically a copy of the first n bytes of the data record but can be any data which will be used to find the record. The key (and hence the record) is locatable via hardware commands.

As known in the art and mentioned above, I/O requests directed to devices in accordance with the CKD format may be in the form of channel programs (also known as channel word programs or chains) including an ordered sequence of channel command words (CCWs); or transport mode (TCW) and the associated device command words (DCWs). Additionally, channel word programs, processing I/O requests, and the like, are described also, for example, in U.S. Pat. No. 6,954,835, INTERCEPTING CONTROL OF A HOST I/O PROCESS, issued Oct. 11, 2005 (the '835 patent); U.S. Pat. No. 6,986,009, INTERCEPTING CONTROL OF A HOST I/O PROCESS, issued Jan. 10, 2006 (the '009 patent); and U.S. Pat. No. 7,707,186, issued Apr. 27, 2010, METHOD AND APPARATUS FOR DATA SET MIGRATION (the '186 patent), all of which are incorporated by reference herein.

The host 110 a may issue a command, such as an I/O command to read or write data to logical device L1 124 of data storage system 102. Generally, the I/O command may be issued over a physical connection, path or channel between host 110 a and data storage 102 where the logical device L1 124 is accessible to the host over such path. The I/O command may be transmitted from the host 110 a and received at a front end adapter or director of the data storage system 102, such as an HA or FA of the data storage system 102. Thus each physical connection, path or channel from the host over which the host accesses the logical device L1 124 may be a path to a particular front end director, such as a particular one of the multiple HAs or FAs of the data storage system 102. In this manner, each of the multiple paths over which the host 110 a may access the logical device L1 124 may be to a different one, or a particular one, of the multiple HAs or FAs of the data storage system 102. In at least some embodiments having multiple paths, normal issuance of an I/O operation may include a host selecting one of the multiple paths based on a suitable technique and associated criteria, such as based on current availability of paths at the time of the I/O, round robin or other performance/load balancing technique, and the like.

Referring to FIG. 4, shown is an example illustrating components and processing that may be performed in an embodiment in accordance with techniques herein. The example 200 includes host 210 and the data storage system 102. The example 200 provides additional detail in connection with only the single data storage system 102 and single host 210 for purposes of simplicity of illustration. However, more generally, such detail described herein with respect to a single data storage system and single host more generally applies to any one or more data storage systems and any one or more hosts. Element 102 may be as described in connection with FIG. 3 including the L1 logical device 124. The host 210 may be a host similar to the hosts 110 a-c and others as described herein. Additionally, the host 210 may include application 212 executing thereon which may issue a write I/O operation 214 that results in modification of data stored at a target location or offset on a logical device such as logical device L1 124.

It should be noted that the write I/O operation 214 may generally denote a modification to any data stored on the logical device at the target location on a logical device. The write operation 214 may be a direct modification of user data, such as a write by the application 212 to update user data stored in a file. Additionally, and more generally, the write operation 214 may denote a modification to user data as well as other types of non-user data stored on the logical device besides user data. Such other types data of the logical device may also include, for example, metadata of the logical device.

Metadata of a logical device that may be modified may include structural information about a data layout of the logical device. For example, the metadata may indicate information such as particular offsets or locations on the logical device where a file system is stored, where each file is stored, where extents or portions of each file are stored, and the like. As a file may increase in size, for example, additional extents may be added to the file whereby such extents of the file may be located at noncontiguous logical offsets or logical addresses of the logical device. In a similar manner, as the file stored on the logical device decreases in size (e.g., such as portions of the file are deleted), the file's metadata stored on the logical device may also change to denote the removed extents. Thus, metadata may be stored at various logical addresses or locations of the logical device where such stored metadata is modified as a result of different operations performed by the application.

In this manner, the write operation 214 may denote generally a write operation that modifies data stored on the logical device whereby the write may modify user data and/or other types of non-user data, such as the metadata, as noted above and elsewhere herein.

The write I/O 214 may result in execution of a sequence of runtime calls or invocations of the I/O path on the host as denoted by the I/O runtime stack 216. Generally, the I/O runtime stack 216 may be characterized as a sequence of layered calls performed to process the write operation 214. Layer 1 may denote the highest layer of the stack 216 and layer N may denote the bottom or lowest layer in the stack 216. As known in the art, the stack 216 may include, for example, a logical volume manager, one or more I/O drivers, and the like. For example, if the write 214 writes user data to a location in a user data file, the stack 216 may include one or more layers that map the particular file location of the user data file to a host side logical device and associated logical device location. Additionally, the stack 216 may include an I/O driver 216 a which may be characterized as a low level I/O driver that, for example, forms I/O command blocks sent to the system 102 and also receives responses from the system 102 in accordance with the particular protocols supported in an embodiment.

In at least one embodiment in accordance with techniques herein, the I/O driver 216 a may perform processing as described herein for I/Os (e.g., read and/or write operations) directed to the L1 logical device 124. Each of the I/Os may be directed to a target location of logical device L1 124. For example, an I/O operation may be a write operation sent 201 from the host 210 to the data storage system 102. The data storage system 102 may write the data of write operation to cache 102 a of the local data storage system 102. Consistent with other discussion herein, the cached write data may be later destaged to physical non-volatile storage provisioned for the L1 logical device 124.

Although in the example 200 of FIG. 4 the I/O operation 214 is a write operation, more generally, the data flow of the I/O path described is applicable for any I/O operation including a read operation with the difference that data is read rather than written with respect to the target location of the logical device.

Referring to FIG. 5, shown is an example 250 illustrating information that may be included in a command request in an embodiment in accordance with techniques herein. The command request 260 may be a request to perform an I/O operation such as may be sent 201 from the host 210 of FIG. 4 to the data storage system 102. The command request 260 may include information such as the I/O command opcode 260 a indicating the I/O operation is a read or write operation, the particular logical address (e.g., the logical device 260 b and location or logical offset(s) 260 c on the logical device) to which the I/O is directed, and the like. The different pieces of information in 260 may be included in various fields of the command request as may vary with the particular layout of the structure for 260 used in an embodiment.

Referring to FIG. 6, shown is an example of multiple engines as may be included in an embodiment of a data storage system in accordance with techniques herein. In this example 700, the data storage system may include a plurality of engines 710 a-710 n. Each of the engines 710 a-710 n may include components thereon as illustrated. In particular, each of the engines may include two directors. Each of the directors may also include two CPU sockets each including a number of “cores” per CPU, and a portion of global memory so that the global memory of the data storage system is collectively all such portions of global memory distributed across the directors of the multiple engines. For example, engine 710 a may include two directors 712 a-b.

Each director of each of the engines 710 a-n may have one or more front end interface connections that support connections to the hosts. Each director may also have one or more back end connections to physical backend storage devices (non-volatile storage devices) to access physical storage devices. In this manner, each director with a front end interface connection may perform processing and function as an HA or FA as described herein. Each director with a connection to backend storage devices (non-volatile storage devices) to access physical storage devices may perform processing and function as a DA as described herein. Additionally, a director may also perform processing and function as an RA as described herein, for example, in connection with remote replication. Thus, a single physical director may perform processing as any one or more of a DA, FA, and/or RA

For example, in at least one embodiment, each of the two directors 712 a-b may be configured to operate as a DA and/or FA as may be desired in a particular configuration such as, for example, to vary the number of DAs and/or FAs in a particular data storage system configuration. For example, in at least one embodiment, each of the directors of each of the engines may be configured to operate as both an FA and a DA so that each single director may both receive front end I/O requests from the host (e.g., FA) and also read data from/write data to physical storage devices (e.g., DA).

Each of the directors 712 a, 712 b of engine 710 a, respectively, may also include a portion of global memory (GM) 714 a,714 b and CPU sockets 715 a, 715 b. Each of the engines 710 a-n may also include components similar to that as illustrated and described with respect to engine 710 a. Directors across the engines 710 a-710 n may communicate over a fabric 750. The fabric 750 may include, for example, a switch and connections between the switch and engines 710 a-710 n. In at least one embodiment, the fabric 750 may be an IB fabric.

The GM portion of each director may be characterized as local with respect to that particular director. For example, director 712 a include GM portion 714 a which is memory that is local to that particular director. Data stored in GM portion 714 a may be directly accessed by a CPU or core of the director 714 a. For example, GM portion 714 a may be memory (e. g., DIMM (dual inline memory module) DRAM (dynamic random access memory)) that is locally accessible by director 714 a where data from one location in 714 a may be copied to another location in 714 a directly using DMA operations (e.g., local memory copy operations) issued by a processor 715 a of director 712 a. Thus, the director 712 a may directly access data of 714 a locally without communicating over the fabric 750 to access global memory. As an alternative, the director 712 a may also use the fabric 750 to access data of 714 a.

GM portion 714 a may include information (as described in more detail below) that is accessed (e.g., for read and/or write) generally by any director of any of the engines 710 a-n. Thus, for example, a director of any of the engines 710 a-710 n may communicate over the fabric 750 to access data in GM portion 714 a. In a similar manner, any director of any of the engines 710 a-n may generally communicate over fabric 750 to access any GM portion comprising the global memory. Although a particular GM portion, such as 714 a may be locally accessible to one of the directors, such as director 712 a, any other director of any engine 710 a-n may generally access the GM portion 714 a. Additionally, the director 712 a may also use the fabric 750 for data transfers to and/or from GM portion 714 a even though 714 a is locally accessible to director 712 a (without having to use the fabric 750).

In at least one embodiment, there may be a maximum of 8 engines and thus 16 directors in a data storage system. The IB fabric may be used generally in embodiments with 1 or more engines (e.g., two or more directors) to facilitate communication and data transfers between directors.

In at least one embodiment of techniques herein using a host based on the IBM® System Z® Architecture and running the z/OS® operating system, there may be a maximum number of 8 physical paths or connections between the host and the data storage system over which a logical device may be accessible to the host (e.g., paths over which the host may issue I/Os to the logical device). Multi-pathing is a term used to refer to the fact that a single logical device may be accessible over multiple paths where multi-pathing is limited by the z/Architecture channel subsystem implementations to 8 channel paths maximum per logical control unit, as noted above.

Discovery processing may be performed with respect to the physical paths over which a logical device on a data storage system is accessible to the host to determine which of the physical paths are online (e.g., active, available, enabled and capable of data transmissions and communication) for use in connection with host-data storage system communications. A logical path mask (LPM) may be determined by the host as a result of the discovery processing. The LPM may identify those physical paths determined by the host as online and over which the logical device is accessible to the host. Thus, the LPM may denote the paths, or that portion of the physical paths, available to the host for sending I/Os to the logical device on the data storage system. Additionally, as part of the discovery processing, the host may determine the particular data storage system director on each of the physical paths or connections. In at least one embodiment, each data storage system director may be uniquely identified using a name, number or other type of identifier that may be vary with embodiment. Various system structures may be used to identify the physical paths and also the LPM over which a logical device is accessible to the host.

In an embodiment in accordance with techniques herein with 4 engines and thus 8 directors configured with front end interfaces that receive I/O operations and requests from an external host (e.g., function as HAs or FAs), each of the 8 channels or paths from the host may connected to a different one of the 8 directors. More generally, for purposes of recoverability, the 8 physical connections or paths (denoting the maximum number of physical channels or paths from the host to the data storage system over which the logical device is accessible) may be distributed among the different directors and engines of the data storage system where each such director receives I/O requests from the host. In embodiments where there are more directors (e.g., 16 directors) than physical paths or connections (e.g., 8) from the host, an embodiment may have each physical path or connection from the host go to a different director although there will not be a physical connection directly to each director/all directors. In embodiments where there are fewer directors than physical paths or connections from the host, an embodiment may have one or more of the directors connected to the host over multiple physical paths or connections. For example, in at least one embodiment, there may be 2 engines and thus 4 directors in the data storage system and there may be 8 physical paths or connections from the host. In such an embodiment, each of the 4 directors of the data storage system may be connected to the host over 2 different physical paths or connections.

An embodiment in accordance with techniques herein may have any suitable maximum number of physical paths or connections between the host and data storage system over which a logical device may be accessed and techniques herein are not limited to use with embodiments have a maximum of 8 such physical connections or paths. An embodiment in accordance with techniques herein may also have any suitable number of directors and/or engines and is not limited to 16 directors as described herein. Additionally, and more generally, the particular limits, maximum values, and other details are provided herein for purposes of illustration and techniques herein may be readily applied for use in systems with other limits, maximum values, configurations, and the like, than as described in examples herein.

In an embodiment in accordance with techniques herein, processing may be performed to select, per I/O, a particular path over which to send the I/O to the logical device. Thus, techniques herein may select one of the available paths indicated by the LPM over which to send each single I/O to the logical device. In such an embodiment, an I/O driver, such as I/O driver 216 a of FIG. 4, may perform such processing to select the particular path per I/O over which the I/O is issued. As described in following paragraphs, such techniques may provide for selecting the path to a particular director of the data storage system communicating with the host, where the selected director (e.g., functioning as an FA) has responsibility for cache slot allocation and locally accessing the cache slot predicted to include the data of the I/O operation (e.g. data stored at the target location to which the I/O operation is directed where the target location may denote a logical device and location (e.g., track) on the logical device).

In such an embodiment, the global memory of the data storage system used as the data cache may be distributed whereby different portions of the distributed global memory, and thus different portions of the data cache, are locally accessible to different directors (e.g., such as described in connection with FIG. 6 and elsewhere herein). Using techniques herein, the I/O driver of the host may send the I/O over a path to a particular director where the particular director selected may have responsibility for cache slot allocation for the particular I/O sent. In such a case, the director selected may also locally access a first portion of the global memory used as the data cache where the first portion (e.g., cache slot of the data cache) is used for storing cached data of the I/O operation received by the director. In at least one embodiment in accordance with techniques herein, an algorithm (also referred to herein as a cache slot allocation algorithm) may be used on the data storage system for determining the particular director responsible for cache slot allocation and caching of data for the I/O operation. The director may cache the I/O operation data in a cache slot of a distributed global memory portion that is local with respect to the director. The algorithm may, for example, take as an input the target address of the I/O operation where the target address denotes the logical device and location on the logical device to which the I/O operation is directed. The algorithm may map the target address of the I/O operation to the particular director responsible for cache slot allocation for I/Os directed to the particular target address. Processing may be performed on the host, such as by an I/O driver of the host, that also has knowledge of the algorithm used on the data storage system for cache slot allocation whereby the host may use the algorithm in connection with determining the particular director expected or predicted to handle cache slot allocation for a particular target address to which the I/O operation is directed. The I/O driver of the host may then specifically send the I/O operation to the particular director over a selected path from the host to the particular director. In this manner, the director receiving the I/O operation may also perform the cache slot allocation and storing of I/O operation data for the received I/O operation. The receiving director may allocate a cache slot location of the distributed global memory for storing the I/O operation data where the cache slot location is included in a portion of the global memory that is local with respect to the receiving director. In this manner, for example, write data directed to a write target address may be received by the director responsible for (and locally accessing) the write target address's predicted cache slot. The write data may be copied by the director into the cache slot as a local copy operation. In contrast, consider the case where a first director receiving the write data from the host is not responsible for the cache slot allocation for the target write address or, more generally, where the cache slot for the target write address is not in a first portion of global memory that is locally accessible to the first director receiving the write data. In this case, the first director may issue a request over the fabric to store the write data in its cache slot location where the cache slot location is included in another second portion of global memory locally accessible to a second different director.

Referring to FIG. 7, shown is an example of information that may be included in a memory map of each director in an embodiment in accordance with techniques herein. The example 300 illustrates a director memory map that may generally including 3 segments or types of information mapped to the director's local physical memory, where at least a portion of the director's local physical memory may form a global memory portion (e.g., 714 a) locally accessible to the director (e.g., 712 a). The memory map 300 may include a first segment 302 of operating system (OS) structures and information, a second segment 304 that include metadata (MD) structures and a third segment 306 of user data stored in cache slots. Collectively, the second MD segments 304 and the third segments 306 of each director's local memory used to cache user data may form the global memory portions (e.g., 714 a-d) of the global memory used as the cache accessible to all directors.

In at least one embodiment, the second segment 304 of the MD structures may include structures that describe the format and structure of the logical devices storing user data. The MD structures of 304 may include, for example, the logical device header and track id (identifier) tables for one or more logical devices. The MD structures of 304 may include the logical device metadata as described elsewhere herein. The third segment 306 may include multiple cache slots each including cached user data and information about the user data cached in the particular cache slot. For example, for each cache slot, information of 306 may include flags denoting state information for the cached data (e.g., whether the cached data is write pending), a logical address on a logical device of the cached data, and the like.

Referring to FIG. 8, shown is an example 500 illustrating a logical representation of data that may be stored in the data storage system cache formed from the different portions of global memory of the different directors in an embodiment in accordance with techniques herein. The example 500 illustrates portions of the distributed cache, and thus distributed global memory, in an embodiment including 16 directors. Column 502 denotes the particular director number or identifier, column 504 denotes the segments of MD structures of the 16 directors, and column 506 denotes the segments of user data cache slots of the 16 directors. Elements 510 a-510 m, respectively, denote the second segments 304 of MD structures stored collectively in the 16 directors (as identified in column 502). Elements 512 a-m, respectively, denote the third segments 306 of the user data cache slots stored collectively in the 16 directors. In this manner, columns 504, 506 respectively denote a logical representation of the aggregated different segments 304, 306 stored in the cache of the data storage system. Each of 510 a-m denotes a portion of the MD structures stored in one of the director's local memory where at least some of the director local memory is used as a global memory portion. For example, element 510 a may denote the MD structures segment 304 of director 1 712 a stored in GM portion 714 a where GM portion 714 a is local memory of the director 1 712 a; element 510 b may denote the MD structures segment 304 of director 2 712 b stored in GM portion 714 b where GM portion 714 b is local memory of the director 2 712 b; and so on with respect to each row of table 500 for a different one of the 16 directors.

In at least one embodiment, MD information for all the logical devices may be stored on (e.g., distributed among) the different GM portions of the different directors. Thus, the collective or aggregated MD structures segments 304 of all directors may comprise the logical device MD for all logical devices. For example, first MD for a first logical device may be stored in 510 a of director 1, and second MD for a second different logical device may be stored in 510 b of director 2.

With reference to columns 504 and 506, the cache slot allocation algorithm used in the data storage system may map a target location of an I/O operation, such as a read or write operation, to a cache slot or cache location. The target location may identify, for example, a logical device and logical address on the logical device to which the I/O operation is directed. The algorithm may map the target location to a particular cache slot or location in cache corresponding to a particular location in the logical cache representation of columns 504 and 506. The particular cache slot or location in cache may then further be mapped to a particular global memory portion including the cache slot where the global memory portion is locally accessible by a single one of the 16 directors. For example, a write I/O operation may write data to logical device L1 at logical address A1 on L1. The algorithm may map (L1, A1) to a cache slot or cache location included in 512 a whereby it may be further determined that 512 a is included in GM portion 714 a that is local to director 1 712 a.

In connection with techniques herein, the foregoing algorithm may be used by the data storage system by each of the directors receiving I/O operations to determine the particular cache slot location used to cache the I/O operation data. If the particular cache slot determined by the algorithm is local to the director that receives the I/O operation, the receiving director may access the cache slot including the I/O operation data using local data transfer operations without requiring use of the IB fabric 750. If the particular cache slot determined by the algorithm is not local to the director that receives the I/O operation, the receiving director may issue a request over the IB fabric 750 to perform any needed data transfers/accesses of the particular cache slot stored in a global memory portion that is local to another one of the directors. In this manner, the host may have knowledge regarding the particular cache slot allocation algorithm used by the data storage system and may also perform the logical processing of the algorithm to predict the expected cache slot and thus particular director responsible for allocation of the expected cache slot (if not already allocated for storing the I/O operation data). The particular director can locally access the I/O operation data of the expected cache slot. If a cache slot is not currently allocated for the I/O operation (such as based on the target location of the I/O operation), the particular director may allocate the cache slot for storing the I/O operation data. If a cache slot is currently allocated for the I/O operation (such as based on the target location of the I/O operation), the particular director does not need to allocate the cache slot and performs local data transfers to and/or from the cache slot (as needed for the particular read or write of the I/O operation data). In at least one embodiment, the I/O driver 216 a of the host 210 of FIG. 4 may predict, using the cache slot allocation algorithm, the particular director as described above. The particular director predicted using the algorithm, for example, is expected to have local access to the allocated cache slot for the I/O operation data (e.g., if I/O operation data is current in cache for the target location of the I/O operation); or the predicted director is otherwise responsible for allocation of such a cache slot for the I/O operation data.

Consistent with discussion herein, an I/O operation (e.g., read or write operation) sent from the host to the data storage system may be directed to a target address identifying a logical address, logical offset or logical location of a logical device. For example, a write I/O operation may write data to a target location of a logical device. In at least one embodiment, the logical device and target location may be expressed using 3 values where one of the 3 values denotes the logical device (e.g., numerical device identifier), and the remaining 2 values identify the target location using a cylinder number and head number. Thus, in at least one embodiment, the logical device number or identifier, cylinder number and head number may denote the target address of an I/O operation. The cache slot allocation algorithm may map the target address to the particular director including a portion of global memory from which the target address's cache slot is expected to be allocated (e.g., in the case of cache misses) or is already allocated (e.g., in case of cache hits). The particular director may handle allocation and storing of cached data for the target address in a cache slot that is local to the particular director. More generally, any algorithm or function may be used to map a target address of an I/O operation expressed in any suitable form (e.g., logical device, logical address or location on the logical device) to the particular director (e.g., such as based on the logical representation of FIG. 8). The algorithm may be used to predict from what global memory portion the cache slot will be allocated and thus which director will locally access the global memory portion including the cache slot. In such an embodiment, the I/O driver on the host may have knowledge of this algorithm used on the data storage system. For example, the logic of the cache slot allocation algorithm used on the data storage system may be embodied in code executed on the host to predict the particular cache slot location, and thus global memory portion and particular director which will locally access the cache slot for a target address of an I/O operation. The I/O driver may then send the I/O operation on a path to the particular director predicted by the algorithm.

In at least one embodiment, more than one cache slot allocation algorithm may be utilized. In such an embodiment, for example, a system call may be used to set or modify the particular cache slot allocation algorithm used. The system call may be made, for example, from the host to the data storage system to thereby allow the host to instruct the data storage system regarding what cache slot allocation algorithm to use. Further, an embodiment including such a system call may allow for identifying the particular cache slot allocation algorithm to use for all logical devices, for one or more specified logical devices, for portions (e.g., extents, tracks or particular logical addresses) of a logical device, and the like. In this manner, the host may, for example, instruct the data storage system to use a first cache slot allocation algorithm for a first logical device (or portion(s) thereof), and a second cache slot allocation algorithm for a second logical device (or portion(s) thereof). In such an embodiment, the particular algorithm may be used to specify a “best effort” in connection with cache slot allocation. It may be, for example, that the algorithm determines director 1 is to handle allocation of a cache slot for caching data stored at logical address or location A1 of logical device L1. However, there may be no free cache slots in director 1's GM portion available for allocation. In such a case, an alternative or different technique may be used to select a cache slot for allocation from another GM portion that is local to another different director. In this manner, the algorithm may be characterized as determining a suggestion for a particular director to handle cache slot allocation and locally access the cache slot including data for a target address of an I/O operation.

With reference back to FIG. 7, a director receiving an I/O operation may store data of the received I/O operation in a buffer referred to herein as an information unit (IU) buffer. Physical memory (local to the director) used for the IU buffer may be mapped into the director's OS system structures segment 302. For example, a write I/O operation that writes data to a target location may be received by a first director. The write data may be stored in an IU buffer in memory local to the first director. Assume there is currently no cache slot allocated for the target address where the cache slot allocation algorithm of the data storage system determines to allocate a cache slot from the global memory portion local to the first director. In this case, the first director performs processing to store the write data from the IU buffer to the allocated cache slot included in the global memory portion that is local to the first director (e.g., allocated cache slot location is included in the memory map of the first director). The foregoing storing of the write data to the allocated cache slot may be performed as a local memory copy operation performed by a processor of the first director without using the fabric. If the allocated cache slot is not local with respect to the first director, the first directory may issue a request over the fabric to write the data a particular allocated cache slot included in another global memory portion that is local with respect to another director.

In a similar manner with reads where read data is stored in the allocated cache slot, the read data may be read from the cache slot location and returned by the first director to the host. In the case where the read data is stored in a cache slot of a global memory portion that is local with respect to the first director, the read data may be obtained by the first director locally accessing the cache slot location in its global memory portion without issuing a request using the fabric. In the case where the allocated cache slot including the read data is not local with respect to the first director that receives the read operation, the first director may issue a request to obtain the requested read data from the allocated cache slot that is local with respect to another second director. The first director may issue the request for the read data over the fabric to the second director. The second director may retrieve the requested read data from the cache slot locally accessible to the second director, and then return the requested read data over the fabric to the requesting first director. The first director may then return the requested read data to the host.

It should be generally noted that in cases for read or write operations directed to a target location where there is no cache slot currently allocated for, or containing data from, the target location, a cache slot may be appropriately allocated and any structures accordingly updated, for example, to set one or more flags of the cache slot location (e.g., such as for a write operation to denote data of the allocated cache slot is write pending), update any appropriate logical device structure(s) to denote that allocated cache slot includes the I/O operation data for a particular logical device and logical address or location of that logical device, and the like.

In at least one embodiment, consistent with discussion elsewhere herein, a director accessing a cache slot in a global memory portion that is local to the director may be performed as a local memory copy operation by a processor of the director. For example, data may be moved between IU buffers of a director to a cache slot included in a global memory portion local to the director by a processor of the director. In such a case, both the IU buffers and the cache slot are included in the memory map or virtual memory address space of the director. However, there may be one or more points in time where the processor utilization of the director may reach a maximum threshold level. In at least one embodiment, when the utilization of the one or more processors of the director exceeds the maximum threshold level of utilization, all reads and writes by the director that, respectively, read and write data from a cache slot in a global memory portion local to the director are performed by issuing requests over the IB fabric. In other words, once the processor (e.g. CPU) utilization of the director reaches the maximum threshold level, all requests to access data (e.g., read and/or write data) in the global memory portion local to the director are issued over the IB fabric. In this case, all requests by the director to access data from any/all global memory portions of the cache (e.g., both local and non-local memory accesses with respect to cache or global memory) may be issued over the IB fabric. In at least one embodiment, the above-mentioned maximum threshold level of CPU utilization may be 65%, selected to be a value in the inclusive range of 65%-67%, or more generally selected to be a value in the inclusive range of 65-70%. The maximum threshold level of CPU utilization may be determined with respect to the collective or aggregate number of cores or processors of the director that may have been designated as available resources to perform the local memory copy operations or tasks of the director.

In at least one embodiment in which the fabric used for communication between directors is an IB fabric, one or more requests to access data stored in the distributed global memory/cache may be performed using a list of structures. For example, consider a case where a director has one or more IU buffers stored in its local memory for request to write data to a cache slot location of global memory. A list of structures may be created that points to the IU buffers of the requesting director. The list may be sent to the IB fabric which extracts the write data from the IU buffers of the requesting director and transmits the extracted data to a particular target director depending on the identified cache slot location that maps to a global memory portion local to the target director. In at least one embodiment, processing may be performed such as by the requesting director to determine which global memory portion of which director includes a particular cache slot location and then direct the request to that particular director. In at least one embodiment, the IB fabric request may be a scheduled asynchronous request. In contrast, the local memory copy or operation performed by a director without using the IB fabric (e.g., copy data from IB buffer to cache slot location) may be performed as a synchronous operation.

Additionally, an embodiment of the data storage system may also select to use the IB fabric rather than have a director perform a local memory operation if the amount of data/size of the data to be transferred exceeds a threshold size. This may be based on the fact that the IB fabric is able to efficiently and quickly perform the data transfer (e.g., read or write data to a cache slot location) even though the director may be able to perform the data transfer as a local memory operation (e.g., both source and target of the data transfer are in memory local to the director). For large amounts of data, in some embodiments, the IB fabric may be able to perform the data transfer faster than if the data transfer is performed as a local memory operation by the director.

An embodiment in accordance with techniques herein may also have the host enable or disable use of techniques herein depending on the size of the I/O operation. For example, the host may determine whether the size of the I/O operation exceeds a specified threshold size. If so, the host may disable processing of techniques herein whereby there is no prediction by the host (using the cache slot allocation algorithm) of the expected cache slot location, and thus no director expected or predicted to locally access the global memory portion including the cache slot location for the I/O operation data. In such a case where techniques herein are disabled on the host side, the I/O operation exceeding the specified threshold size may be sent generally on any selected path using any suitable technique. Responsive to determining the I/O operation exceeds a maximum size, the I/O operation may be sent over a path to any one of a plurality of directors of the data storage system selected independent of which particular director of the plurality of directors is predicted by the cache slot allocation algorithm to locally access the first cache slot. By default, techniques herein may be enabled for an I/O operation to predict the cache slot location used for storing the I/O operation data and thus predict the particular director that locally accesses the predicted cache slot location. Using such techniques herein when enabled, the host (such as an I/O driver on the host) may send the I/O operation over a path to the particular director predicted or expected to have local access to the global memory portion including the cache slot used for storing the I/O operation's data (where the cache slot is that as predicted using the cache slot allocation algorithm). In at least one embodiment, the host may enable or disable use of techniques herein based on feedback information provided by the data storage system to the host. For example, the data storage system may provide information to the host where the information may include CPU or processor utilization (for CPU or processors of the data storage system) as measure of how busy or saturated the data storage system is. Described elsewhere herein is use of CPU utilization on the data storage system to possibly disable use of the techniques herein. In a similar manner, such information may be provided by the data storage system to the host where the host (rather than the data storage system) makes the decision of whether to enable or disable use of techniques herein.

In at least one embodiment, the host may perform processing to partition a single I/O operation into multiple smaller I/O operations. An application on a host may issue an I/O operation, such as a write operation, to a target location on the logical device, where the target location denotes a logical address portion (e.g., range or target area) that spans multiple cache slots when stored in a data cache on the data storage system. Such multiple cache slots may be predicted using the cache slot allocation algorithm to be local to different directors of the data storage system. In such instances, the host I/O driver may receive the single host write operation and further partition the single host write operation into multiple write operations where each of the multiple write operations may write an amount of data no larger than the size of a single cache slot on the data storage system. Each of the multiple I/O operations may also be sent by the host I/O driver to one of the directors predicted or expected to locally access the cache slot of the particular I/O operation data (e.g. the cache slot being predicted using the cache slot allocation algorithm as described herein). Each of the multiple write operations may individually be transmitted to the data storage system over a particular path to a particular director in accordance with the cache slot allocation algorithm as executed on the host. Consistent with discussion herein, the algorithm may be executed on the host for a write operation directed to a target location on a logical device to predict the particular director which is expected or predicted to locally access the cache slot for the target location of the logical device. For example, a single host write operation may write data to a target logical address range on a first logical device where the target logical address range spans two cache slots on the data storage system. In this case, the I/O driver may determine that the single host write spans a target range (e.g., writes an amount of data) that spans two cache slots. The I/O driver may partition the single host I/O write into two smaller write operations including a first write operation that writes data to the first logical device target locations A1 through An stored in the first cache slot, and a second write operation that writes data to the first logical device target locations B1 through Bn stored in the second cache slot. The first write operation may be sent over a path to a first director which is expected or predicted to locally access the first cache slot including data for the first logical device target locations A1 through An. In a similar manner, the second write operation may be sent over a path to a second director which is expected or predicted to locally access the second cache slot including data for the first logical device target locations B1 through Bn. The I/O driver may perform the foregoing partitioning in a manner transparent to the issuing application and issue the multiple smaller write operations to the data storage system 102 rather than a single write operation. In such an embodiment, the I/O driver may perform additional processing to track the status of each of the smaller I/O operations since each such write operation may be completed and performed in any order. More generally, the I/O driver may partition a single originating host write operation into any suitable number of smaller write operations (e.g., writing to a different logical address range) whereby collectively the smaller write operations specify a collective or aggregate set of target logical addresses equivalent to that of the single originating host write operation. It should be noted that the foregoing example mentions partitioning a single host write operation into multiple write operations. However, more generally, the foregoing applies for use in connection with host read operations as well.

Referring to FIG. 9, shown is an example of information that may be used by an I/O driver of the host in an embodiment in accordance with techniques herein. The example 400 illustrates information that may be maintained and used by the I/O driver in connection with tracking the multiple I/O operations created as a result of a single originating host I/O operation such as issued by an application executing on the host. In at least one embodiment, each originating host I/O that is further partitioned into multiple smaller I/O operations may be assigned a unique identifier (ID) used to track and uniquely identify the originating host I/O. The table 400 may include a first column 402 denoting the originating host I/O ID, a second column 404 denoting the multiple smaller I/Os created from the single host write I/O, and a third column 406 denoting the status of each of the smaller I/Os. The table may include a set of information or entry for each of the originating I/Os. For example, row 407 a includes information for the single host I/O having ID=A for which the I/O driver created two smaller I/Os—A1 and A2. In this example as denoted by 407 a in column 406, write A1 is still pending while I/O A2 has completed. Row 407 b includes information for another single host I/O having ID=B for which the I/O driver created 3 smaller I/Os—B1, B2 and B3. In this example as denoted by 407 a in column 406, I/Os B1 and B2 are pending while I/O B2 has completed.

A status of pending in column 406 may denote that particular smaller I/O operation has not yet completed. For example, for a smaller write operation, a pending status in column 406 denotes that the I/O driver on the host has not yet received an acknowledgement from the data storage system that the smaller write operation has completed. A status of completed in column 406 may denote that particular smaller write operation has completed in that the I/O driver on the host has received an acknowledgement from the local or first data storage system 102 that the smaller write operation has completed. Once the I/O driver receives notice of completion of all smaller writes 404 created for a particular originating write denoted by 402, then the I/O driver may return an acknowledgement to the application or other requester that the originating write has completed. The I/O driver may also update the table 400 to remove information regarding an originating write once all created writes for the originating write have completed.

In a similar manner for a smaller read operation, a pending status in column 406 denotes that the host has not yet received the requested read data for the smaller read operation. Once the I/O driver receives notice of completion of all smaller reads 404 created for a particular originating read denoted by 402, then the I/O driver may return an acknowledgement to the application or other requester that the originating read has completed. The I/O driver may also update the table 400 to remove information regarding an originating read once all created smaller reads (denoted in column 404) for the originating read (in column 402) have completed.

Described above are embodiments in accordance with techniques herein to perform I/O path optimizations, per I/O operation, based on predicted cache slot location. As noted above, an embodiment includes a host that directs I/O down channel(s) or path(s) that are connected to the director that has the responsibility for the cache slot allocations for the I/O (e.g., extent/tracks accessed by the I/O). In such an embodiment, local data for the I/O operation stored in the IU buffer of the director may be copied into the cache slot as a local copy rather than as a fabric request.

In at least one embodiment of a data storage system configuration described above, each director has local memory (e.g., DIMM DRAM) that includes system structures, MD and user cache slots (e.g., see FIGS. 7 and 8 and descriptions herein). Each Director may also have front-end interface connections that support host connections and also back-end interface connections to physical storage devices of the back-end providing non-volatile data storage. Additionally, an embodiment of the data storage system may include multiple engines, each with two directors, where such engines/director may be connected by a fabric, such as the IB fabric. Host channels or paths to a logical device may span directors/engines. When an I/O is sent by the host over a channel and the cache slot (e.g., resides in GM portion of DRAM on the director) that needs to be accessed or allocated for this I/O request is locally resident with respect to the receiving director, a local copy can be initiated. A local copy can be performed when the IU and the cache slot are both in the same director where such a local copy may have an advantage in terms of time and resources over the alternative of performing the copy/data transfer over the fabric to another director in a multi-engine configuration. The local copy trades the reduced latency of the memcopy (memory copy) operation (e.g., as may be performed for data transfers between the IU and cache slot in local memory of the same director) especially for small data transfers. However, performing such local copies by a director in at least one embodiment causes an increase in CPU or processor utilization of the director since the memcopy operation is a function performed by instruction on a director core. When the CPU utilization of a director reaches a maximum threshold level, the local memcopy operation may be bypassed and alternatively the fabric may be used for the data transfer, as discussed above. It is also important to note that on very large transfers that span multiple cache slots, in some embodiments it may be faster to send the data over the Fabric rather than perform the data transfer using a memcopy/local memory copy operation. In embodiments including a majority of relatively small (e.g., 2K-27K byte) data transfers, larger performance benefits may be realized using the memcopy/local memory copy operation when possible such as in accordance with techniques herein. It should be noted that the use of memcopy herein generally refers to a memory copy operation between a source and target memory location. In terms of the C programming language, for example, the memcopy may be expressed as “memcpy”.

As described herein the host may generally understand or have knowledge regarding the cache slot allocation algorithm used on the data storage system and how the cache slots are allocated relative to a logical device and logical addresses or locations (e.g., the track locations) on that device. Having this information, the host may direct a specific I/O down the path to a director that has, or will allocate, the cache slots to be used for storing cached data of the I/O. An embodiment in accordance with techniques herein may use a cache slot allocation algorithm used on the host side to predict, for an I/O operation, what data storage system director either has the cache slot allocated (e.g., data for the I/O operation currently in cache for the device/track to which the I/O operation is directed) or would be responsible for the cache slot allocation.

Generally, techniques herein for the I/O path optimization of directing and I/O on a path to a particular director expected to locally access the cache slot for the I/O and handle cache slot allocation for the I/O may be performed with respect to any I/O operation directed to a target location, such as a logical address, track, location, and the like, on a logical device where the I/O operation data is or will be stored in a cache slot of the distributed global memory on the data storage system. In particular, for example, such techniques may be performed with respect to an I/O operation that reads and/or writes user data such as may be stored in a cache slot mapped to a segment 306 of a director. Such techniques herein may also be performed with respect to an I/O operation that reads and/or writes other data, such as MD for the logical devices stored in an cache slot mapped to segment 304 of a director.

Referring to FIG. 10, shown is a flowchart of processing steps that may be performed in an embodiment in accordance with techniques herein. The flowchart 600 generally summarizes processing described above where the host I/O driver performs the I/O path optimization for an I/O operation. At step 602, the host I/O driver receives an I/O operation directed to a target location (e.g., logical device and location on a logical device). At step 604, processing is performed to determine, using the cache slot allocation algorithm utilized on the data storage system, a director of the data storage system. The director is predicted, using the algorithm, to handle allocation of the cache slot that stores for the I/O operation data for the target location. The director is configured to locally access the cache slot included in a global memory portion that is local with respect to the director. At step 606, processing is performed, such as by the I/O driver of the host, to select a path or channel from the host to the data storage system where the selected path or channel is the predicted director determined in step 604. In step 608, processing is performed to send the I/O operation from the host to the data storage system over the selected path or channel to the predicted director. At step 610, the director receives the I/O operation. At step 612, processing is performed on the data storage system to service the I/O operation. Such processing of step 612 may include, for example, allocating any cache slot(s) needed for storing the I/O operation data of the target location, and performing any local and/or non-local (over the IB fabric) data transfers to service the I/O operation (e.g., including data transfers of the I/O operation data in and out of cache). As described herein, if the cache slot including the I/O operation data of the target location is local with respect to the receiving director (that received the I/O operation in step 510), the director may perform local data transfers to and/or from the cache slot. If the cache slot including the I/O operation data of the target location is not local with respect to the receiving director (that received the I/O operation in step 510), the director may perform non-local data transfers to and/or from the cache slot using the IB fabric. Depending on the particular CPU utilization, size of the data transfers, and the like, the director may also perform processing to forego any possible local data transfers to and/or from the cache slot and may alternatively select to perform all data transfers over the IB fabric. At step 614, once the I/O operation has been serviced, the director (that received the I/O operation in step 510) may return an acknowledgment and any requested data to the host in connection with the I/O operation serviced.

Although not explicitly noted in FIG. 10, the host I/O driver may perform other processing as described herein depending on the particular embodiment. For example, the host I/O driver may disable performing the optimization for I/O operations having an I/O size larger than a specified threshold, may partition a single write into multiple smaller writes, and the like, as described above.

An embodiment may implement the techniques herein using code executed by a processor. For example, an embodiment may implement the techniques herein using code which is executed by a processor. As will be appreciated by those skilled in the art, the code may be stored on a computer-readable medium having any one of a variety of different forms including volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules or other data. Computer-readable media includes, but is not limited to, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, (DVD) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can accessed by a processor.

While the invention has been disclosed in connection with preferred embodiments shown and described in detail, their modifications and improvements thereon will become readily apparent to those skilled in the art. Accordingly, the spirit and scope of the present invention should be limited only by the following claims. 

What is claimed is:
 1. A method of processing an I/O operation comprising: receiving, on a host, the I/O operation directed to a target location of a logical device having storage provisioned on a data storage system; predicting, in accordance with a cache slot allocation algorithm used by the data storage system, a first cache slot of a cache of the data storage system, wherein the first cache slot is expected to be used to store cached data for the target location of the logical device; determining, by the host, a director of the data storage system, wherein the director locally accesses the first cache slot of the cache of the data storage system; and sending the I/O operation from the host to the data storage system over a path, said path being a physical connection from the host to the director of the data storage system, wherein said predicting, said determining and said sending are performed responsive to the I/O operation being a request that reads or writes a first amount of data that does not exceed a maximum size, and wherein responsive to determining the I/O operation exceeds a maximum size, sending the I/O operation to any one of a plurality of directors of the data storage system selected independent of which particular director of the plurality of directors is predicted by the cache slot allocation algorithm to locally access the first cache slot.
 2. The method of claim 1, wherein said predicting and said determining are performed by an I/O driver of the host.
 3. The method of claim 1, wherein the target address denotes a logical address or location on the logical device.
 4. The method of claim 3, wherein the cache slot allocation algorithm maps the logical device and logical address or location on the logical device to which the I/O operation is directed to the first cache slot, and wherein the first cache slot is included in a first global memory portion that is local with respect to the director.
 5. The method of claim 4, wherein the data storage system includes a plurality of directors comprising the director, the data storage system further including a distributed global memory comprising a plurality of global memory portions including the first global memory portion, each of the plurality of directors locally accessing a different one of the plurality of global memory portions, and wherein the cache of the data storage system comprises groups of multiple cache slots, each of the groups of multiple cache slots being included in a different one of the global memory portions.
 6. The method of claim 5, wherein each of the plurality of directors is configured to communicate over a fabric to access the distributed global memory.
 7. A method of processing an I/O operation comprising: receiving, on a host, the I/O operation directed to a target location of a logical device having storage provisioned on a data storage system; predicting, in accordance with a cache slot allocation algorithm used by the data storage system, a first cache slot of a cache of the data storage system, wherein the first cache slot is expected to be used to store cached data for the target location of the logical device; determining, by the host, a director of the data storage system, wherein the director locally accesses the first cache slot of the cache of the data storage system; and sending the I/O operation from the host to the data storage system over a path, said path being a physical connection from the host to the director of the data storage system, wherein the target address denotes a logical address or location on the logical device, wherein the cache slot allocation algorithm maps the logical device and logical address or location on the logical device to which the I/O operation is directed to the first cache slot, and wherein the first cache slot is included in a first global memory portion that is local with respect to the director, wherein the data storage system includes a plurality of directors comprising the director, the data storage system further including a distributed global memory comprising a plurality of global memory portions including the first global memory portion, each of the plurality of directors locally accessing a different one of the plurality of global memory portions, and wherein the cache of the data storage system comprises groups of multiple cache slots, each of the groups of multiple cache slots being included in a different one of the global memory portions, wherein each of the plurality of directors is configured to communicate over a fabric to access the distributed global memory, and wherein each of the plurality of directors is configured to send all requests to access the distributed global memory over the fabric if processor utilization of said each director exceeds a maximum threshold utilization level.
 8. The method of claim 7, further comprising: determining, by a first of the plurality of directors, that processor utilization of said first director exceeds the maximum threshold utilization level; and responsive to determining that processor utilization of the first director exceeds the maximum threshold utilization level, issuing a request by said first director over the fabric to access first data stored in a first location of one of the plurality of global memory portions that is locally accessible to the first director.
 9. The method of claim 8, further comprising: returning, over the fabric to said first director, first data stored in the first location of the one global memory portion that is locally accessible to the first director.
 10. The method of claim 6, wherein each of the plurality of directors is configured to send, over the fabric, requests that transfer an amount of data exceeding a defined maximum amount, wherein the requests access locations in the distributed global memory.
 11. A method of processing an I/O operation comprising: receiving, on a host, the I/O operation directed to a target location of a logical device having storage provisioned on a data storage system; predicting, in accordance with a cache slot allocation algorithm used by the data storage system, a first cache slot of a cache of the data storage system, wherein the first cache slot is expected to be used to store cached data for the target location of the logical device; determining, by the host, a director of the data storage system, wherein the director locally accesses the first cache slot of the cache of the data storage system; and sending the I/O operation from the host to the data storage system over a path, said path being a physical connection from the host to the director of the data storage system, wherein the target address denotes a logical address or location on the logical device, wherein the cache slot allocation algorithm maps the logical device and logical address or location on the logical device to which the I/O operation is directed to the first cache slot, and wherein the first cache slot is included in a first global memory portion that is local with respect to the director, wherein the data storage system includes a plurality of directors comprising the director, the data storage system further including a distributed global memory comprising a plurality of global memory portions including the first global memory portion, each of the plurality of directors locally accessing a different one of the plurality of global memory portions, and wherein the cache of the data storage system comprises groups of multiple cache slots, each of the groups of multiple cache slots being included in a different one of the global memory portions, wherein each of the plurality of directors is configured to communicate over a fabric to access the distributed global memory wherein each of the plurality of directors is configured to send, over the fabric, requests that transfer an amount of data exceeding a defined maximum amount, wherein the requests access locations in the distributed global memory, and wherein the method further comprises: determining, by a first of the plurality of directors, that a first request to access a first location in the distributed global memory transfers a first amount of data exceeding the defined maximum amount, wherein the first location denotes a global memory location of the distributed global memory and the global memory location is locally accessible to the first director; and responsive to determining that the first amount exceeds the defined maximum amount, issuing the first request by the first director over the fabric.
 12. The method of claim 11, further comprising: returning, over the fabric to the first director, first data stored in the first location denoting the global memory location.
 13. A method of processing an I/O operation comprising: receiving, on a host, the I/O operation directed to a target location of a logical device having storage provisioned on a data storage system; predicting, in accordance with a cache slot allocation algorithm used by the data storage system, a first cache slot of a cache of the data storage system, wherein the first cache slot is expected to be used to store cached data for the target location of the logical device; determining, by the host, a director of the data storage system, wherein the director locally accesses the first cache slot of the cache of the data storage system; and sending the I/O operation from the host to the data storage system over a path, said path being a physical connection from the host to the director of the data storage system, wherein the target address denotes a logical address or location on the logical device, wherein the cache slot allocation algorithm maps the logical device and logical address or location on the logical device to which the I/O operation is directed to the first cache slot, and wherein the first cache slot is included in a first global memory portion that is local with respect to the director, wherein the data storage system includes a plurality of directors comprising the director, the data storage system further including a distributed global memory comprising a plurality of global memory portions including the first global memory portion, each of the plurality of directors locally accessing a different one of the plurality of global memory portions, and wherein the cache of the data storage system comprises groups of multiple cache slots, each of the groups of multiple cache slots being included in a different one of the global memory portions, wherein each of the plurality of directors is configured to communicate over a fabric to access the distributed global memory, and wherein the method further comprises: determining whether the I/O operation accesses data spanning multiple cache slots of the cache of the data storage system, wherein each of the multiple cache slots is locally accessible to a different one of the plurality of directors; responsive to determining the I/O operation accesses data spanning multiple cache slots each locally accessible to a different one of the plurality of directors, partitioning the I/O operation into multiple I/O operations wherein each of the multiple I/O operations accesses data of a different one of the multiple cache slots; and issuing each of the multiple I/O operations accessing said different one of the multiple cache slots over a particular path to one of the plurality of directors that locally accesses said different one of the multiple cache slots.
 14. A non-transitory computer readable medium comprising code stored thereon that, when executed, performs method of processing an I/O operation comprising: receiving, on a host, the I/O operation that reads or writes first metadata of a logical device having storage provisioned on a data storage system; predicting, in accordance with an algorithm used by the data storage system, a first cache slot of a cache of the data storage system, wherein the first cache slot is expected to be used to store the first metadata for the logical device; determining, by the host, a director of the data storage system, wherein the director locally accesses the first cache slot of the cache of the data storage system; and sending the I/O operation from the host to the data storage system over a path, said path being a physical connection from the host to the director of the data storage system, wherein said predicting, said determining and said sending are performed responsive to the I/O operation being a request that reads or writes a first amount of data that does not exceed a maximum size, and wherein responsive to determining the I/O operation exceeds a maximum size, sending the I/O operation to any one of a plurality of directors of the data storage system selected independent of which particular director of the plurality of directors is predicted by the cache slot allocation algorithm to locally access the first cache slot.
 15. A system comprising: a processor; and a memory comprising code stored thereon that, when executed, performs a method of processing an I/O operation comprising: receiving, on a host, the I/O operation that reads or writes first data stored on a data storage system; predicting, in accordance with an algorithm used by the data storage system, a first cache slot of a cache of the data storage system, wherein the first cache slot is expected to be used to store the first data; determining, by the host, a director of the data storage system, wherein the director locally accesses the first cache slot of the cache of the data storage system; and sending the I/O operation from the host to the data storage system over a path, said path being a physical connection from the host to the director of the data storage system, wherein said predicting, said determining and said sending are performed responsive to the I/O operation being a request that reads or writes a first amount of data that does not exceed a maximum size, and wherein responsive to determining the I/O operation exceeds a maximum size, sending the I/O operation to any one of a plurality of directors of the data storage system selected independent of which particular director of the plurality of directors is predicted by the cache slot allocation algorithm to locally access the first cache slot.
 16. A non-transitory computer readable medium comprising code stored thereon that, when executed, performs a method of processing an I/O operation comprising: receiving, on a host, the I/O operation directed to a target location of a logical device having storage provisioned on a data storage system; predicting, in accordance with a cache slot allocation algorithm used by the data storage system, a first cache slot of a cache of the data storage system, wherein the first cache slot is expected to be used to store cached data for the target location of the logical device; determining, by the host, a director of the data storage system, wherein the director locally accesses the first cache slot of the cache of the data storage system; and sending the I/O operation from the host to the data storage system over a path, said path being a physical connection from the host to the director of the data storage system, wherein the target address denotes a logical address or location on the logical device, wherein the cache slot allocation algorithm maps the logical device and logical address or location on the logical device to which the I/O operation is directed to the first cache slot, and wherein the first cache slot is included in a first global memory portion that is local with respect to the director, wherein the data storage system includes a plurality of directors comprising the director, the data storage system further including a distributed global memory comprising a plurality of global memory portions including the first global memory portion, each of the plurality of directors locally accessing a different one of the plurality of global memory portions, and wherein the cache of the data storage system comprises groups of multiple cache slots, each of the groups of multiple cache slots being included in a different one of the global memory portions, wherein each of the plurality of directors is configured to communicate over a fabric to access the distributed global memory, and wherein each of the plurality of directors is configured to send all requests to access the distributed global memory over the fabric if processor utilization of said each director exceeds a maximum threshold utilization level.
 17. A system comprising: a processor; and a memory comprising code stored thereon that, when executed, performs a method of processing an I/O operation comprising: receiving, on a host, the I/O operation directed to a target location of a logical device having storage provisioned on a data storage system; predicting, in accordance with a cache slot allocation algorithm used by the data storage system, a first cache slot of a cache of the data storage system, wherein the first cache slot is expected to be used to store cached data for the target location of the logical device; determining, by the host, a director of the data storage system, wherein the director locally accesses the first cache slot of the cache of the data storage system; and sending the I/O operation from the host to the data storage system over a path, said path being a physical connection from the host to the director of the data storage system, wherein the target address denotes a logical address or location on the logical device, wherein the cache slot allocation algorithm maps the logical device and logical address or location on the logical device to which the I/O operation is directed to the first cache slot, and wherein the first cache slot is included in a first global memory portion that is local with respect to the director, wherein the data storage system includes a plurality of directors comprising the director, the data storage system further including a distributed global memory comprising a plurality of global memory portions including the first global memory portion, each of the plurality of directors locally accessing a different one of the plurality of global memory portions, and wherein the cache of the data storage system comprises groups of multiple cache slots, each of the groups of multiple cache slots being included in a different one of the global memory portions, wherein each of the plurality of directors is configured to communicate over a fabric to access the distributed global memory, and wherein each of the plurality of directors is configured to send all requests to access the distributed global memory over the fabric if processor utilization of said each director exceeds a maximum threshold utilization level.
 18. A non-transitory computer readable medium comprising code stored thereon that, when executed, performs a method of processing an I/O operation comprising: receiving, on a host, the I/O operation directed to a target location of a logical device having storage provisioned on a data storage system; predicting, in accordance with a cache slot allocation algorithm used by the data storage system, a first cache slot of a cache of the data storage system, wherein the first cache slot is expected to be used to store cached data for the target location of the logical device; determining, by the host, a director of the data storage system, wherein the director locally accesses the first cache slot of the cache of the data storage system; and sending the I/O operation from the host to the data storage system over a path, said path being a physical connection from the host to the director of the data storage system, wherein the target address denotes a logical address or location on the logical device, wherein the cache slot allocation algorithm maps the logical device and logical address or location on the logical device to which the I/O operation is directed to the first cache slot, and wherein the first cache slot is included in a first global memory portion that is local with respect to the director, wherein the data storage system includes a plurality of directors comprising the director, the data storage system further including a distributed global memory comprising a plurality of global memory portions including the first global memory portion, each of the plurality of directors locally accessing a different one of the plurality of global memory portions, and wherein the cache of the data storage system comprises groups of multiple cache slots, each of the groups of multiple cache slots being included in a different one of the global memory portions, wherein each of the plurality of directors is configured to communicate over a fabric to access the distributed global memory wherein each of the plurality of directors is configured to send, over the fabric, requests that transfer an amount of data exceeding a defined maximum amount, wherein the requests access locations in the distributed global memory, and wherein the method further comprises: determining, by a first of the plurality of directors, that a first request to access a first location in the distributed global memory transfers a first amount of data exceeding the defined maximum amount, wherein the first location denotes a global memory location of the distributed global memory and the global memory location is locally accessible to the first director; and responsive to determining that the first amount exceeds the defined maximum amount, issuing the first request by the first director over the fabric.
 19. A system comprising: a processor; and a memory comprising code stored thereon that, when executed, performs a method of processing an I/O operation comprising: receiving, on a host, the I/O operation directed to a target location of a logical device having storage provisioned on a data storage system; predicting, in accordance with a cache slot allocation algorithm used by the data storage system, a first cache slot of a cache of the data storage system, wherein the first cache slot is expected to be used to store cached data for the target location of the logical device; determining, by the host, a director of the data storage system, wherein the director locally accesses the first cache slot of the cache of the data storage system; and sending the I/O operation from the host to the data storage system over a path, said path being a physical connection from the host to the director of the data storage system, wherein the target address denotes a logical address or location on the logical device, wherein the cache slot allocation algorithm maps the logical device and logical address or location on the logical device to which the I/O operation is directed to the first cache slot, and wherein the first cache slot is included in a first global memory portion that is local with respect to the director, wherein the data storage system includes a plurality of directors comprising the director, the data storage system further including a distributed global memory comprising a plurality of global memory portions including the first global memory portion, each of the plurality of directors locally accessing a different one of the plurality of global memory portions, and wherein the cache of the data storage system comprises groups of multiple cache slots, each of the groups of multiple cache slots being included in a different one of the global memory portions, wherein each of the plurality of directors is configured to communicate over a fabric to access the distributed global memory wherein each of the plurality of directors is configured to send, over the fabric, requests that transfer an amount of data exceeding a defined maximum amount, wherein the requests access locations in the distributed global memory, and wherein the method further comprises: determining, by a first of the plurality of directors, that a first request to access a first location in the distributed global memory transfers a first amount of data exceeding the defined maximum amount, wherein the first location denotes a global memory location of the distributed global memory and the global memory location is locally accessible to the first director; and responsive to determining that the first amount exceeds the defined maximum amount, issuing the first request by the first director over the fabric.
 20. A non-transitory computer readable medium comprising code stored thereon that, when executed, performs a method of processing an I/O operation comprising: receiving, on a host, the I/O operation directed to a target location of a logical device having storage provisioned on a data storage system; predicting, in accordance with a cache slot allocation algorithm used by the data storage system, a first cache slot of a cache of the data storage system, wherein the first cache slot is expected to be used to store cached data for the target location of the logical device; determining, by the host, a director of the data storage system, wherein the director locally accesses the first cache slot of the cache of the data storage system; and sending the I/O operation from the host to the data storage system over a path, said path being a physical connection from the host to the director of the data storage system, wherein the target address denotes a logical address or location on the logical device, wherein the cache slot allocation algorithm maps the logical device and logical address or location on the logical device to which the I/O operation is directed to the first cache slot, and wherein the first cache slot is included in a first global memory portion that is local with respect to the director, wherein the data storage system includes a plurality of directors comprising the director, the data storage system further including a distributed global memory comprising a plurality of global memory portions including the first global memory portion, each of the plurality of directors locally accessing a different one of the plurality of global memory portions, and wherein the cache of the data storage system comprises groups of multiple cache slots, each of the groups of multiple cache slots being included in a different one of the global memory portions, wherein each of the plurality of directors is configured to communicate over a fabric to access the distributed global memory, and wherein the method further comprises: determining whether the I/O operation accesses data spanning multiple cache slots of the cache of the data storage system, wherein each of the multiple cache slots is locally accessible to a different one of the plurality of directors; responsive to determining the I/O operation accesses data spanning multiple cache slots each locally accessible to a different one of the plurality of directors, partitioning the I/O operation into multiple I/O operations wherein each of the multiple I/O operations accesses data of a different one of the multiple cache slots; and issuing each of the multiple I/O operations accessing said different one of the multiple cache slots over a particular path to one of the plurality of directors that locally accesses said different one of the multiple cache slots.
 21. A system comprising: a processor; and a memory comprising code stored thereon that, when executed, performs a method of processing an I/O operation comprising: receiving, on a host, the I/O operation directed to a target location of a logical device having storage provisioned on a data storage system; predicting, in accordance with a cache slot allocation algorithm used by the data storage system, a first cache slot of a cache of the data storage system, wherein the first cache slot is expected to be used to store cached data for the target location of the logical device; determining, by the host, a director of the data storage system, wherein the director locally accesses the first cache slot of the cache of the data storage system; and sending the I/O operation from the host to the data storage system over a path, said path being a physical connection from the host to the director of the data storage system, wherein the target address denotes a logical address or location on the logical device, wherein the cache slot allocation algorithm maps the logical device and logical address or location on the logical device to which the I/O operation is directed to the first cache slot, and wherein the first cache slot is included in a first global memory portion that is local with respect to the director, wherein the data storage system includes a plurality of directors comprising the director, the data storage system further including a distributed global memory comprising a plurality of global memory portions including the first global memory portion, each of the plurality of directors locally accessing a different one of the plurality of global memory portions, and wherein the cache of the data storage system comprises groups of multiple cache slots, each of the groups of multiple cache slots being included in a different one of the global memory portions, wherein each of the plurality of directors is configured to communicate over a fabric to access the distributed global memory, and wherein the method further comprises: determining whether the I/O operation accesses data spanning multiple cache slots of the cache of the data storage system, wherein each of the multiple cache slots is locally accessible to a different one of the plurality of directors; responsive to determining the I/O operation accesses data spanning multiple cache slots each locally accessible to a different one of the plurality of directors, partitioning the I/O operation into multiple I/O operations wherein each of the multiple I/O operations accesses data of a different one of the multiple cache slots; and issuing each of the multiple I/O operations accessing said different one of the multiple cache slots over a particular path to one of the plurality of directors that locally accesses said different one of the multiple cache slots. 